Combustible gases in the presence of oxygen create a hazardous mixture; mines, tankers and many industrial processes are exposed to or handle such mixtures. Industrial safety requires the monitoring and control of such mixtures in such environments. The energy industry on the other hand combusts such mixtures under control to create energy and is also interested in the monitoring and control of such mixtures for optimising efficiency.
Devices based on the monitoring of the change in the resistance of a temperature dependent and catalytic element and relating this change to the concentration of a combustible gas which is catalytically combusted on the element, have been available for many years. The simplest is just a heated platinum wire forming a part of a wheatstone bridge. The `Pellistor` is probably the most common form of this type of device (U.S. Pat. Nos. 3,092,799/63, 3,200,011/65, 3,564,474/71). In its basic form it consists of a platinum coil which acts both as a heater and as a temperature sensor. The coil is encapsulated by a refractory pellet of porous alumina in which a catalyst is dispersed in one element. The platinum coil heats the catalyst to a suitable temperature at which the oxidation of the combustible gas is induced on the surface of the catalyst. The heat generated will be conducted to the coil and raise its temperature and hence alter its resistance. A coil in a pellet with no catalyst is used as a reference.